The present invention relates to a DC-DC converter including a blocking oscillating circuit and a voltage resonance type switching circuit.
A DC-DC converter of the type in which a first DC voltage is converted into a voltage with a rectangular wave by means of a switching circuit, the rectangular wave voltage is smoothed through a transformer, and the smoothed voltage is supplied to a load, as a second DC voltage, has been known. This type of DC-DC converter, because of meritorious features of small size and light weight, has widely been used as DC power sources for terminal equipment of computers and general communication equipment. This is especially true for a small capacity DC-DC converter with an output of 200 W or less of the single end type in which a switching element for turning on and off a first DC voltage is inserted between one end of the transformer and a first DC voltage source, because of its low cost and simple construction. For further details of the single end type DC-DC converter, reference is made to IEEE Power Electronic Conference, 1977, pp 160, "A New Optimum Topology Switching DC-DC converter". A basic switching scheme in the DC-DC converter is categorized into a buck power stage system, a boost power stage system, a buck-boost power stage system, and a boost-buck power stage system. Common to those systems is a conversion of the DC power source voltage into a voltage having a rectangular wave. The common feature increases a switching loss of the switching element making up the switching circuit to necessitate a radiator plane for radiating heat. A self-exciting type circuit with omission of an oscillating circuit for driving the switching circuit, employed in the switching system, hinders the increase of the switching speed by a feedback action. A voltage resonance type switching system as a modification of the buck-boost power stage system as mentioned above, allows the improvement of a switching efficiency by shaping "L" the switching characteristic of the switching element. The "L shaping" of the switching characteristic means that in the graph in which a current of the switching element is plotted on an ordinate and a voltage between terminals of the switching element is plotted on an abcissa the switching element is operated to form the voltage-current characteristic of "L" Shape. This approach indeed succeeds in increasing the switching efficiency, but needs an external drive circuit and an auxiliary power source for driving the switching element, resulting in the complexity of the construction of a DC-DC converter. In this respect, there has been a demand for driving the switching element by the output of an oscillating circuit driven by the first DC voltage mentioned before. The attempt, however, has been unsuccessful in that the mere use of an oscillating circuit provides an unstable or abnormal oscillation output due to a counter action of the switching circuit.